A vehicle speed control system is conventionally known in the art, in which vehicle speed is automatically reduced when vehicle deceleration is necessary.
According to the conventional system of this kind, for example, as disclosed in Japanese Patent Publication No. 1994-36187, a curve approaching speed “vt” is calculated based on geographical information from a vehicle navigation system in order that a vehicle can safely pass through a curve ahead thereof, and vehicle speed “v” is automatically decreased to such calculated curve approaching speed “vt”. More exactly, deceleration, which is necessary for constantly (at a constant deceleration) reducing the vehicle speed “v” at a curve starting point to the curve approaching speed “vt”, is calculated based on a distance “d” to the curve starting point and the current vehicle speed “v”. When the calculated deceleration exceeds a predetermined maximum permissible deceleration “Gmax”, the vehicle is automatically controlled in such a manner that the vehicle is decelerated at such maximum permissible deceleration “Gmax”.
According to another vehicle speed control system, as disclosed in Japanese Patent Publication No. 1998-269498, maximum permissible deceleration “Gmax” is changed depending on a vehicle driver's intention (driving ability).
According to the above conventional vehicle speed control system, as shown in FIG. 8, an automatic decelerating operation is started, when a vehicle approaches a curved road at a constant vehicle speed “v0” from its initial condition S(d0, v0), and when the vehicle condition reaches a “Gmax” curve (namely, when the vehicle condition crosses the “Gmax” curve from a right side of the curve). As a result, the vehicle speed “v” is reduced along the “Gmax” curve, so that the vehicle speed “v” becomes to a curve approaching speed “vt” when the vehicle comes to a curve starting point (d=0). This is a vehicle condition “St”.
According to the above conventional system, however, an operation for the following case is not taken into consideration. Namely, it is the case, in which a braking operation is carried out by a vehicle driver during the automatic decelerating operation, and the deceleration required by such braking operation of the vehicle driver is larger than the maximum permissible deceleration “Gmax” (namely, the brake override occurs).
In the case that the brake override occurs, it may be considered to cancel the automatic decelerating operation, as in a similar manner to a conventional cruise control operation. In such a case, however, the automatic decelerating operation must be carried out again, when the vehicle deceleration by the braking operation of the vehicle driver is not sufficient after the cancellation of the automatic decelerating operation. This kind of operation is not disclosed in the above conventional system. Accordingly, the present invention has an object to provide a vehicle speed control system, according to which a smooth vehicle deceleration is realized even in the case the brake override occurs.
More exactly, as indicated by a dotted line in FIG. 9A, if the brake override occurs after the automatic decelerating operation at the maximum permissible deceleration “Gmax” has started at the vehicle condition “S1” (in which the vehicle is decelerated along the “Gmax” curve), condition for the automatic decelerating operation drops out (the vehicle deceleration is departed from the “Gmax” curve). However, when the vehicle deceleration by the braking operation of the vehicle driver is not sufficient, the vehicle condition reaches again the “Gmax” curve at the vehicle condition “S2”. As a result, the automatic decelerating operation at the maximum permissible deceleration “Gmax” is carried out again.
A control torque changes as indicated by a solid line in FIG. 9B during the above operation of the vehicle deceleration. Namely, when the automatic decelerating operation is started at the vehicle condition “S1”, the control torque is controlled at a constant torque amount “τmax” which corresponds to the maximum permissible deceleration “Gmax” (the vehicle condition is “S11”). When a braking torque amount “τd” (indicated by a dotted line) required by the braking operation of the vehicle driver becomes larger than the constant torque amount “τmax” (indicated by a dotted thin line) for the automatic decelerating operation, namely, when the brake override occurs at a vehicle condition “S12”, the automatic decelerating operation is cancelled, wherein the torque amount by the automatic decelerating operation becomes zero (as indicated by a one-dot chain line). Instead, the required braking torque amount “τd” is used as the control torque. When the required braking torque amount “τd” is thereafter decreased as a result that the braking force (i.e. brake pedal stepping force) by the vehicle driver is made smaller, and thereby when the vehicle condition reaches again the “Gmax” curve, as shown in FIG. 9A (vehicle condition “S2”), the automatic decelerating operation is started again. Namely, as indicated by the one-dot-chain line in FIG. 9B, the torque amount is again controlled at the torque amount “τmax” (at a vehicle condition “S21”). As above, in the case that the vehicle is not sufficiently decelerated by the braking force of the vehicle driver after the vehicle override has occurred, the vehicle deceleration (the control torque) for the automatic operation changes intermittently and largely. As a result, it may cause a problem that an uncomfortable feeling (a large deceleration) is possibly given to the vehicle driver, even though the vehicle driver does not strongly step the brake pedal.